PEGMATITE, SILEXITE, AND APLITE OF NEW YORK 37 



A typical example bringing out the relation of the earlier to 

 the later pegmatites occurs in a big ledge on the little hill just 

 south of the mouth of Two Brooks near Goldsmith. Much of 

 the granite contains streaks, lenses, and eyes, usually less than an 

 inch wide and not over two feet long, of not very coarse pegmatite 

 parallel to the foliation. This pegmatite makes up a very con- 

 siderable percentage of the rock mass, with contacts varying from 

 fairly sharp to not sharp. Cutting this combination of granite 

 and pegmatite are several prominent later pegmatite dikes, six to 

 eight feet wide and forty or fifty feet long, carrying equal amounts 

 of quartz and feldspar and 10 per cent of biotite. These moderately 

 coarse-grained later pegmatite dikes lie only partly roughly parallel 

 to the granite foliation. This ledge calls to mind an exposure ob- 

 served by the writer in the Blue Mountain quadrangle,^ where lenses 

 and eyes of pegmatite parallel to the foliation of a mixed gneiss 

 are sharply cut across by a pegmatite dike of distinctly later origin. 



On a little ridge one mile east-northeast of Goldsmith an out- 

 crop shows two bands or long lenses of pegmatite parallel to the 

 foliation of the granite and without very sharp contacts. The 

 upper lens consists of two-thirds quartz and one-third feldspar, 

 and it is very uniform. The lower lens is notably variable, with 

 the different portions not sharply separated. Nearly pure silica 

 (silexite) and typical pegmatite here developed side by side, while 

 the inclosing granite was still somewhat fluid, as shown by the 

 flow-structure along the lower side of the lens. 



Near the road two miles east-southeast of Goldsmith a peg- 

 matite dike, or rather a segregation mass, one foot wide and twelve 

 feet long, which lies parallel to the foliation of the inclosing granite 

 and grades perfectly into it, shades off into a zone of nearly pure 

 silica (silexite) four to five inches wide toward its middle. Both 

 the pegmatite and the silexite here developed side by side by 

 segregation during a late stage in the solidification of the granite 

 magma, and both pegmatite and silexite now occupy practically 

 the place where they solidified. 



A more exceptional mode of occurrence of the older pegmatites 

 is illustrated in a ledge by an old road one mile northwest of the 



' W. J. Miller, N.Y. State Mus., Bull. ig2 (1916), pi. 10. 



